Many transmission systems employ modulation of a carrier signal in order to convey data. For example, Frequency Modulation (FM) based systems modulate the frequency of a carrier signal in dependence upon an input voltage level. Frequency Shift Keying (FSK) is a particular implementation of the FM scheme, and involves shifting the frequency of the carrier between two discrete frequencies.
In order to recover data from an FM signal, a receiver must have access to the carrier frequency used by the transmitter. A demodulator typically mixes the received signal with the carrier frequency to recover the baseband signal. A demodulator scheme for FSK is described, for example, in Kaveh Shakeri, Hossein Hashemi, Ali Parsa, Ali Fotowat, Reza Rofougaran: “A 1 Volt CMOS 2/4-level FSK Digital Demodulator for Pager Applications”.
Whilst it is possible to provide a clock at the receiver which operates at a frequency similar to the clock used by the transmitter, some automatic frequency correction (AFC) mechanism is required at the receiver in order to “lock” the receiver clock to the transmitter clock. AFC mechanisms usually consist of a clock and data recovery loop, forming a phase locked loop (PLL) that aligns rising edges of the clock to be in the middle of a data bit (of a demodulated data stream). The system has to be able to work in the presence of noise, which will introduce jitter to the edge positions of the data stream. In a typical receiver architecture, the AFC is a discrete block located after the demodulator as is illustrated in FIG. 1.